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Welcome to Ask Tom!, a monthly column by our resident water treatment guru, Tom Keenan of
National Environmental Services Agency (NESA). Tom addresses the issues that bug you the most. And Tom knows!! With 35 years experience in providing environmental support services to public and private sector clients on a wide range of environmental issues. Tom has also co-authored and presented training courses on wastewater treatment systems.
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In Control - Considerations for
Control Systems
Part Two: Flow Measurement
by Dan Capano, DTS, Inc.
Flow is one of the most common
process parameters to be measured. Flow measurement choices are
numerous, and the application of the correct instrument can have
far-reaching effects on system operation and budget. This month I
will briefly explain the most common options for flow measurement.
Each type of flowmeter is unique in the method used to measure
flow. The variety of instruments available allows the application
of an instrument with the best fit for a wide range of process
fluids, environments and locations.
Nature of Flow
Flow is defined as the movement of
a fluid. To measure flow rate is to measure the volume of fluid
moving through or past a physical point at a given time.
The flowmeters described below
measure flow volumetrically, as opposed to Mass Flowmeters which
measure the quantity of mass passing through or past a given
point. It is desirable to continuously measure the rate of flow of
a process stream to allow operators or engineers to determine the
state of an operation or process. Flow rate is used to determine
if a system is operating within capacity and to automate processes
such as the addition of some chemical to the process stream. An
example would be the addition of chlorine to plant effluent for
disinfection. Monitoring of flow allows planners and engineers to
properly assess needs and design systems of the proper capacity or
production.
Different methods are used to
measure flow. Each method was developed to provide accuracy with
minimal process disturbance. Factors to consider when installing
or replacing a flowmeter are:
- Maintenance: Consider
what demands the new device will place on your maintenance
resources. If the device must be cleaned, adjusted, filled or
overhauled, then adequate resources must be committed to
maintain meter accuracy. Also, consider if a pipeline must be
discharged or a process interrupted in order to repair or
calibrate the instrument.
- Proper matching: Matching
flowmeter to the process is crucial. If a process stream
contains large amounts of solids or debris, restrictions or
obstructions then an improperly applied flowmeter will
eventually fail. If the meter material is incompatible with
the process fluid, premature failure can also be expected. A
failure of this type could also endanger personnel. Proper
instrument enclosure selection protects personnel, equipment
and facilities from damage
- Power considerations:
Every installation is different. Remote locations may operate
on low voltage loops or may not be electrified at all.
- Cost: Flowmeter cost
varies directly with complexity for a given installation. Shop
around. Don't overlook bargains in the used equipment market.
- Range and Accuracy:
Flowmeters are available in virtually any range. Accuracy is
usually stated as of percentage of full scale. The term
turndown ratio is used to describe the range over which a
flowmeter will perform at its rated accuracy. A 10: turndown
means that a flowmeter having a range of 0-100 GPM is accurate
from about 10 GPM through maximum. Repeatability is a term
used to describe the percentage of change of successive
measurements.
Types of Flowmeters
Differential Pressure producers are
a class of flowmeters used extensively. Flowmeters of this type
rely on the development of a measurable differential pressure to
function. Placing an obstruction at some point in the process
stream will artificially produce a Differential Pressure (DP). The
pressure before and after this obstruction is measured and
compared; the flow rate is calculated from the resulting number.
Obstructions in the line interfere with the flow of the process
fluid and will cause a loss of system pressure. To maintain flow
and pressure, energy in the form of increased flow must be added
to the system. This is accomplished by increased pump or
compressor cycles with a commensurate rise in energy consumption.
It is desirable, in most cases, to minimize the effect of the
obstruction, which is exhibited in loss of head in the system.
Some devices produce more head loss than others; these are, in
order of rising pressure drop:
 Pitot
Tubes: These devices use small diameter concentric tubes to
measure upstream and downstream pressures. The tubes are inserted
into the stream with the high-pressure port facing upstream and
the low measuring static pressure. The impact pressure of the
process stream at the high-pressure tube produces a measurable
rise in pressure relative to the static pressure measurement
point(s). A very small target is presented to the moving fluid and
the tube produces a negligible effect on the system pressure.
These devices require 10-30 diameters of straight pipe upstream
and 5-15 downstream to avoid the effects of turbulence on the
meter. The tubes can become fouled and their use in other than
clean gases or liquids is not recommended. While reasonably
accurate over the middle range of the instrument, accuracy is
limited at low flows.
Venturi Tubes: These devices
are inserted into the pipe and become part of the piping system.
The upstream side of the device slowly tapers to a narrow throat.
Downstream from the throat, the device flares out again, allowing
near recovery of lost system pressure. Converging (upstream)
angles are between 19 and 23 degrees, 21 degrees being the most
common angle used. The diverging (downstream) angle is typically
7.5 degrees, but up to 15-degree divergence angles are used with
some sacrifice to system pressure recovery. Venturis require up to
ten pipe diameters of straight pipe upstream to avoid turbulence
effects from the system. Downstream requirements are not critical
to proper operation. This type of DP device presents a moderate
head loss to the system and can be used for a wide variety of
process fluids in both the liquid and gas phases. Venturis are
maintenance free and are cost effective over a large range of
flow. Most venturis are designed to return 90 % of lost pressure
to the system.
 Orifices:
These devices employ a flat plate with a precisely ground orifice
which may be concentric, eccentric or segmental. The orifice is
usually mounted between two pipe flanges. Measurement ports may be
integral to the flanges, or may be at some predetermined distance
(diameters) upstream and downstream of the orifice. Requirements
for straight pipe upstream vary with the ratio of the orifice
diameter to the pipe diameter and can exceed fifty pipe diameters.
Straight pipe requirements can be reduced with the use of
straightening vanes or tubes. Orifices produce the greatest head
loss, and are not suitable for slurries or dirty fluids and their
use is limited in corrosive duty because of erosion of the orifice
edge. Orifice flowmeters are used chiefly for high-pressure gas or
liquid applications.
 Magnetic
Flowmeters: These devices rely on magnetism and Faraday's Law
of Electromagnetic Induction to provide flow measurement with no
obstruction and pressure loss. In it's simplest form, a magmeter
is a spool piece (pipe with two flanges) wound with a coil of
wire. Two or more electrodes are positioned on the insides of the
pipe wall. If the coil is energized, a magnetic field will be
developed in the tube. As a process fluid is passed through the
tube, voltage is induced and passed to the electrodes. The level
of induced voltage is directly proportional to the flow rate.
Magmeters are always supplied as a "full-bore" device,
meaning that the instrument measures the full diameter of the
pipe.
The major disadvantage of this type
of meter is the unavoidable process interruption required for
meter repair or replacement. Magmeters require at least five pipe
diameters upstream and three downstream. A Magmeter requires a
full pipe to provide the best accuracy; if the meter electrodes
are exposed or partially submerged, the meter will operate
erratically, if at all. These meters are available in a wide range
of sizes and liner materials and because there is no obstruction
in the pipeline, cause no pressure drop in the system. A variety
of process fluids and slurries can be accommodated. The meter can
be configured as bi-directional models allowing flow measurement
in both directions.
 Turbine
Meters: Turbine meters operate by placing a multi-bladed
propeller into the process stream. The flow of liquid impinges
upon the blades, causing it to rotate. A magnetic pickup senses
this rotation; the speed at which the propeller rotates is
directly proportional to flow rate. Turbine meters offer two types
of installation: insertion and full-bore. Full bore meters are
constructed similarly to the magmeters described above. Turbine
meters can be rendered unusable by a poor installation. Turbulence
at the meter resulting from installation close to bends will
adversely affect meter operation; valves or other upstream
disturbances will also cause erratic operation. Cavitation is
another concern and will result in erratic operation and possibly
damage the meter over a long period of time. The meter must be
protected from damage by any debris and a screen may be required
upstream for this purpose. Total pressure drop across turbine
meters is typically in the range of 3-6 PSIG at full flow. Turbine
meters require at least fifty diameters of straight pipe upstream
in order to avoid this problem. In order to avoid this
requirement, flow-straightening devices are used to reduce the
requirement to a manageable ten diameters upstream and downstream.
Turbine Meters may be used for clean liquids and gases and are
available in bi-directional models allowing flow measurement in
both directions.
Vortex Flowmeters: These
are devices that produce vortices within the meter body in order
to measure flow rate. Two methods are used to develop and measure
vortices. The first is Vortex Shedding. In this method, a
"bluff body" is placed into the process stream. The
impact of the fluid upon the bluff body causes shedding of
vortices alternately from either side causing the pressure across
the bluff body to alternate from high to low pressure. As a vortex
forms on one side, fluid velocity increases, lowering pressure;
the opposite side of the bluff body experiences a rise in pressure
with a drop in fluid velocity. The frequency of vortex shedding is
proportional to the fluid flow rate. Vortex shedders are used in
all types of clean service for liquids and gases. Upstream pipe
diameters vary with the meter, but range from 5 to 150 diameters
upstream. As always, straightening vanes or tubes can be used to
reduce this requirement. Downstream requirements are usually 5
diameters.
Vortex precession meters utilize
spiral guide vanes to produce a controlled swirl in the process
stream, causing a swirling vortex to form. As the vortex moves
through the meter, it remains cohesive and presents a higher fluid
velocity at a given point in the meter at a period proportional to
the fluid flow rate. This change in velocity is sensed using
thermistors or piezoelectric sensors. The amount of heat removed
from a heated thermistor is measured. As heat is removed, the
heater voltage changes and can be measured. The amount of heat
removed is proportional to and dependent on the fluid flow rate.
Because of the high tolerances required when machining the meter
bodies, these meters are more expensive than other types of
meters. Vortex flowmeters are comparable in accuracy to other
types of meters, but are limited to use in clean fluids and gases.
 Ultrasonic
flowmeters: These use ultrasonic energy to measure flow
rate. Aside from open-channel flow measurement, which will be
discussed below, two types of ultrasonic meters are in common use.
The first type is known as a Transit Time Flowmeter and operates
by measuring the time for an ultrasonic pulse to traverse a pipe
section both with and against the process flow. The fluid flow
adds and subtracts a velocity component to the transit times;
these times are calculated and return a flow rate. This type of
meter is available as a "clamp-on" where the transducers
are clamped around an existing pipeline. Transit time meters are
also available as "spool pieces" which are inserted into
the line and contain integral transducers. Transit Time meters
require a full pipe to operate correctly and must be installed
with 10-20 straight pipe diameters upstream and 5 diameters
downstream. Solids are tolerated by these devices, but bubbles in
the process stream will cause erratic or unusable readings. In
order to provide a more reliable signal, particularly in larger
pipe diameters, multiple transducers with multiple acoustic paths
are used. Bi-directional models are also available for flow
measurement in either direction.
Doppler Flowmeters rely on
the presence of bubbles and solids in the process stream in order
to function properly. Most manufacturers will specify the lower
limits of solids concentration or size required for the particular
meter to operate. The device operates by bouncing an ultrasonic
pulse off of a moving particle. The reflected pulse is shifted
according to the Doppler principle. Another consideration for
proper operation is the requirement that flow be sufficient to
keep particles in suspension. Typically 10-20 straight pipe
diameters are needed upstream with 5 diameters downstream.
Transducers are mounted on the outside of the pipe and are
available in one or two transducer models. Two transducer models
"shoot" an ultrasonic pulse through the pipe against the
flow.
Ultrasonic meters are
relatively inexpensive and reasonably accurate. Proper
installation is crucial because of the nature of the measurement
technique. Improper alignment of the transducers on either type of
instrument will result in almost certain headaches. This type of
flowmeter allows measurement without process interruption,
however, and allow portability.
Open Channel Flow Measurement
In open channel flow measurement,
weirs and flumes are used to produce known and repeatable
characteristics in the process stream. Weirs, flumes and other
devices develop a liquid head that is used to measure flow rate.
These devices are particularly useful when large flows are to be
measured or where liquids are handled in open channels or in
pipelines that are not ordinarily full. Flows from 100,00 GPM to
the millions of gallons per day are measured using these devices.
 Weirs
are the simplest and most economical flow-measuring devices
available. Basically, weirs are obstructions or dams placed across
the channel containing openings or apertures of known geometry
through which flows the liquid to be monitored. Weirs may be
rectangular, trapezoidal (Cippoletti), "V" notch or a
combination of these types. Head is measured as the rise in level
of liquid in the pool upstream of the weir. V notch weirs are used
for smaller flow rates than those handled by rectangular or
trapezoidal weirs. All produce known characteristics in the
process stream that can be measured to indicate flow rate. Weirs
use staff gauges mounted on the side of the weir box to provide a
measurement of head. Ultrasonic level instruments are commonly
used to measure the rise in head. The use of a stilling well
should be considered if foaming or excessive turbulence is
encountered.
Flumes are slightly more
complex devices. Flumes use complex geometries to form the process
stream into a controlled flow that allows accurate and repeatable
measurement over the range of the flume. The loss of head through
a flume is about one-quarter that of a weir of equal capacity.
Flumes have been around since the 1920's. The most common flume is
the Parshall Flume, named for Dr. R. L. Parshall, who developed it
for use in irrigation. A Parshall Flume installed in a pre-fabricated
manhole. Note the ultrasonic transducer over the stilling well on
the left.
A
Parshall Flume installed in a pre-fabricated manhole.
Note the ultrasonic transducer over the stilling well on the left.
The Parshall flume places a
constriction on the stream and then drops the stream at the throat
of the flume to produce a repeatable, cohesive surface to provide
a flow measurement. The surface of the stream at the throat rises
and falls linearly with the flow rate. A staff gauge is usually
affixed to the inside surface of the flume for reference. A
stilling well may also be provided, allowing the use of an
additional electronic measuring device such as an ultrasonic or
admittance type meter. Advantages of flumes are the ability to
self-clean, low head loss and wide operating range. Several
different type of flumes are in use, including:
- Palmer-Bowlus
- Leopold-Lagco
- Cutthroat Flume
Mass flowmeters, rota-meters and
specialized flow instruments are not included in this discussion
and will be explained in a future column. The reader is
invited to pursue further reading on the subject of flow
measurement. The Internet is a wealth of information on the
subject. A good starting point is the bureau of reclamation's
website, which features a full text version of their flow
measurement manual. The Bureau's website is:
http://www.usbr.gov/wrrl/fmt/wmm/
Other useful references are ISCO,
Inc.'s "Open Channel Flow Measurement Handbook". ISCO is
located in Lincoln, NE. As always, I welcome any questions,
comments or observations from readers.
Next month: Level Measurement
Methods and Devices.
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